1. Field of the Invention
The invention in general relates to ferroelectric memories having a thin film of ferroelectric layered superlattice material with a thickness of 90 nanometers or less, and to a method of fabricating such thin films.
2. Statement of the Problem
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possesses desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Layered superlattice material oxides have been studied for use in integrated circuits. See Watanabe, U.S. Pat. No. 5,434,102. Layered superlattice materials exhibit characteristics in ferroelectric memories that are orders of magnitude superior to alternative types of ferroelectric materials, such as PZT and PLZT compounds. Integrated circuit devices containing ferroelectric elements with layered superlattice materials are currently being manufactured. The layered superlattice materials comprise metal oxides.
It is highly desirable that a ferroelectric memory be dense; that is, that there be a high number of memory cells in a given chip volume. To achieve maximum density, the individual elements of the memory should be as small as possible. This requires that the films of ferroelectric material be as thin as possible.
Nevertheless, it is generally known in the art that as a ferroelectric film is made thinner, its critical electronic properties, particularly the ferroelectric polarizability, deteriorate. See Batra et al., "Phase Transition, Stability, and Depolarization Field in Ferroelectric Thin Films", Physical Review, Vol. 8, No. 7, pp. 3257-3265 (October 1973), at page 3261, bottom of first column and top of second column, FIG. 4, and page 3265, last sentence of Section IV. Conclusion. The theoretical analysis of Batra et al. has been shown to be correct by experiment. See, for example, Robert W. Vest and Jiejie XU, "PbTiO.sub.3 Films From Metalloorganic Precursors", IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol, 13, No. 6, November 1998, page 711, col. 1, first paragraph, and page 714, last paragraph. See also U.S. Pat. No. 5,519,234, issued May 21, 1996 to Carlos A. Paz De Araujo et al., particularly FIG. 25 and the discussion at col. 34, lines 28-33. The polarizability of a ferroelectric memory must be at least seven microcoulombs per centimeter squared (.mu.C/cm.sup.2) to make a practical memory. See, for example, European Patent Publication No. 0 489 519 A2, page 4, lines 3-7, and page 5, lines 7-10. As indicated in the Vest article and U.S. Pat. No. 5,519,234 references above, the polarizability of ferroelectric materials generally falls below this level when made less than 140 nanometers to 200 nanometers thick. Therefore, when utilizing thin films of ferroelectric materials, generally several coatings of ferroelectric material are made to build up the thickness to about 140 nanometers (1400 .ANG.) or higher, so that the polarizability will be sufficiently high to make a memory. See U.S. Pat. No. 5,198,269, issued March 30, 1993, to Scott L. Swartz and Peter J. Melling.
An additional obstacle to fabricating dense ferroelectric memories has been the difficulty of making extremely thin films on commercially practical substrates. Generally, it has been found that the processing parameters, such as annealing temperatures, necessary to produce integrated circuit quality electronic devices also cause films less than about 100 nm to crack or othewise fail.
To increase the density of ferroelectric memories, it would be highly desirable to have a ferroelectric thin film with a polarizability greater than seven .mu.C/cm.sup.2 and a thickness significantly less than about 100 nm, and which can be fabricated using a commercially feasible process.